Sound attenuation apparatus

ABSTRACT

Apparatus for reducing acoustic transmission from mechanical or acoustic sources by means of a double wall partition, within which an acoustic pressure field is generated by at least one secondary acoustic source. The secondary acoustic source is advantageously placed within the partition, around its edges, or it may be an integral part of a wall of the partition.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, as amended, Public Law85-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of sound barriers and in particularto a sound barrier capable of attenuating sound over a range offrequencies.

2. Description of Related Art

It is well known in the field of sound barriers that a single wall willreduce the sound transmitted by a source. In order to achieve thedesired degree of sound attenuation, it may be necessary to use a wallso heavy or stiff as to be impractical, as for example, a wall onboardor as part of an aircraft or space vehicle, where it is desired tominimize weight. Further, the wall thickness required increases as thesound frequency decreases. Thus, especially at low frequencies,particularly thick and heavy walls are required.

One improvement to the use of a single wall is a double wall or othercompound wall structure, which may have the capability of equivalentsound attenuation at lower total weight. The double or compound wall mayalso be stiffened, or filled with a material which absorbs or dampssound, such as a fiberglass mat or blanket; also, visco-elastic dampingmaterials may be applied to the walls.

However, even compound walls with acoustic treatment may not besuitable. U.S. Pat. No. 4,600,078 teaches the use of acousticalresonators with a double wall partition as a means of attenuating sound.Each resonator may be designed for a specific frequency.

The use of resonators inherently imposes limitations which it is anobject of the present invention to overcome.

Accordingly, it is a primary object of the present invention to providean improved sound barrier.

It is another object of the present invention to provide a sound barrierwith increased sound attenuation effectiveness without increasing thetotal weight thereof. It is a further object of the present invention toprovide a sound barrier which conveniently attenuates sound of varyingfrequency, or sound of simultaneous multiple frequencies.

It is still another object of the present invention to provide a soundbarrier which incorporates acoustical elements.

It is a further object of the present invention to provide a soundbarrier suitable for the construction of fuselage walls for surfacevehicles, marine/submarine craft, aircraft, and space vehicles.

SUMMARY OF INVENTION

According to the present invention, a double wall partition is placedbetween a primary acoustic source which it is desired to attenuate and areceiver zone. Within the partition is a sound-transmitting medium suchas air. At least one secondary acoustic source, which may be aloudspeaker, is used to generate an acoustic pressure field within thepartition, which has the effect of reducing sound in the receiver zone.

It is not an essential element of this invention that the secondaryacoustic source(s) be within the double wall partition. Indeed, thesecondary acoustic source(s) may be external to the double wallpartition. However, the secondary acoustic source(s) or its (their)mountings may be an integral part of at least one wall of the partition.Where the secondary acoustic source is a loudspeaker, it must beenclosed, and the enclosure may be an integral part of a wall or of thepartition perimeter.

The frequency of the primary and secondary acoustic source(s) must besubstantially identical. It may be necessary, depending on thecharacteristics of the primary source, to adjust the amplitude and phaseof the sound generated by the secondary source(s). Thus, the inventionhas its broadest application where the frequency, amplitude and phase ofthe secondary source(s) are adjustable.

Depending on the characteristics of the primary source, a greater degreeof sound attenuation may be achieved for a given secondary acousticsource depending on its location with respect to, or within, thepartition. It is especially advantageous if the secondary source isreadily movable, especially by a means external to the partition.

It is especially beneficial to have one or more sensing devices in thereceiver zone or within the partition to measure sound level and tocommunicate with an automatic control device to vary the characteristicsof the sound generated by the secondary acoustic source(s), viz., thefrequency, amplitude and phase thereof, or to change the location of thesecondary acoustic source(s), so as to readily and continuously minimizethe sound perceived in the receiver zone.

Especially good results are obtained when the spacing between the wallsof the partition is less than the wavelength of the sound to beattenuated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, includingits objects and benefits, reference should be made to the Description ofthe Preferred Embodiments, which is set forth below. This descriptionshould be read together with the accompanying drawings, wherein:

FIG. 1A is a schematic showing a side view of one embodiment of a soundbarrier according to the present invention;

FIG. 1B is a schematic showing nine measurement positions in a verticalplane parallel to the sound barrier of FIG. 1A and spaced 24 inchestherefrom;

FIG. 2A is a schematic showing a side view of another embodiment of asecond barrier according to the present invention; and

FIG. 2B is a schematic showing the positions of four loudspeakers withinthe sound barrier of FIG. 2A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention may be understood by reference to the following examplesand their reference figures.

EXAMPLE 1

Referring now to FIG. 1A, two rooms, a source room and a receiving room,were separated by a common concrete wall 3 which had a 48"×60" openingtherein. A double wall partition 4 was installed in the opening, made ofthe following materials: wall 5 was 0.063 inches thick aluminum and wall6 was 0.25 inches thick plexiglass.

A loudspeaker was placed in the source room, to serve as primary source1, located 18-inches from partition 4. Source 1 generated a pure tonesignal at a frequency of 125 Hz. In the receiving room, a microphone 7was placed 24-inches from wall 6 and 18-inches below the horizontalcenterline of partition 4.

The signal generated by source 1 was measured at 84 dB by microphone 7.

A closepacked cluster of four juxtaposed loudspeakers 2A, 2B, 2C, and 2Dwas located at the center of the double wall partition to comprisesecondary source 2. A 125 Hz signal was fed to the cluster, and theamplitude and phase were adjusted until the sound detected by microphone7 was at a minimum. The minimum sound level measured by microphone 7 was47 dB when the phase difference between sources 1 and 2 was 164°.

EXAMPLE 2

Using the same room and conditions as in Example 1, noise reduction wasmeasured at nine different locations (8, 9, 10, 11, 12, 13, 14, 15, and16) in a vertical plane containing microphone 7 as shown in FIG. 1B.With source 1 and source 2 at the identical frequencies, amplitudes andrespective phases as in Example 1, an average sound pressure level forthe nine locations was measured first when source 1 alone was operating,and then when sources 1 and 2 were operating together at the identicalfrequency, amplitudes and phases as in Example 1. When sources 1 and 2were operated, the average sound pressure level was decreased by 8.2 dB,compared to only source 1 operating.

EXAMPLE 3

Example 1 was repeated, but with microphone 7 moved to a location 17,which is 60-inches from the partition rather than 24-inches. At location17, the measured sound pressure level went from 88 dB without source 2to 49 dB with source 2 operating; the phase difference between signalsto source 1 and 2 was 161°. This demonstrates that a noise reduction isobtained at varying distances from the partition.

EXAMPLE 4

Example 3 was repeated, using the same location 17 for the microphone,but at different frequencies. Results follow:

    ______________________________________                                        Frequency           200 Hz  400 Hz                                            ______________________________________                                        Source 1 only, measured dB                                                                        86      77                                                Source 1 and source 2                                                                             49      42                                                operating, dB                                                                 Phase difference, degrees                                                                         220°                                                                           344°                                       ______________________________________                                    

This Example shows that noise is reduced at different frequencies.

EXAMPLE 5

Example 1 was repeated, but the four loudspeakers of source 2 wereplaced in the four corners within the double wall partition 4. See FIGS.2A and 2B.

Results follow:

    ______________________________________                                        Frequency           125 Hz                                                    ______________________________________                                        Source 1            84 dB                                                     Source 1 + Source 2 58 dB                                                     Phase Difference, degrees                                                                         291°                                               ______________________________________                                    

This Example shows that the source 2 location affects resultsquantitatively, but in all cases studied, there is some noise reduction.

What is new and desired to be secured by Letters Patent of the UnitedStates is:
 1. A sound barrier comprising:a multiple wall partitionpositioned between a primary acoustic source producing a first sound ofa frequency and a zone wherein the sound of said primary acoustic sourceis to be attenuated; a sound-transmitting medium within said partition;and at least one secondary acoustic source positioned to generate asecond sound within the partition, the second sound having a frequency,magnitude, and phase selected so as to achieve attenuation in said zone.2. The sound barrier of claim 1, wherein the multiple wall partition isa double wall partition.
 3. The sound barrier of claim 2 wherein the atleast one secondary acoustic source is placed within the double wallpartition.
 4. The sound barrier of claim 2 wherein the at least onesecondary acoustic source is external to the double wall partition. 5.The sound barrier of claim 2 wherein the at least one secondary acousticsource is an integral part of at least one of the walls of thepartition.
 6. The second sound barrier of claim 2 wherein the secondsound generated by the at least one secondary acoustic source may beadjusted with respect to a member of the group consisting of: frequency,amplitude, and phase.
 7. The sound barrier of claim 6 wherein at leastone sound-sensing device is placed in the receiver zone or within thepartition and said at least one sound-sensing device is used toautomatically vary a member of the group consisting of frequency,amplitude, and phase of the second sound generated by the at least onesecondary acoustic source so as to maximize the desired soundattenuation.
 8. The sound barrier of claim 3 wherein said at least onesecondary acoustic source is readily movable by a means external to thepartition.